Book - Developmental Anatomy 1924-8

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Chapter VIII The Urogenital System

The urinary and reproductive systems are associated intimately in development. Both arise from the mesoderm of the intermediate cell mass fnephrotome), which unites the primitive segments with the lateral layers of somatic and splanchnic mesoderm (Figs. 35 S and 128). In the course of development these anlages bulge into the coelom as paired longitudinal ridges, termed the urogenital folds (Figs. 131, 143 and 144).

Vertebrates possess excretory organs of three district types. The pronephros is the functional kidney of amphioxus and certain lampreys, but appears in immature fishes and amphibians only to be replaced by the mesonephros. The embryos of amniotes (reptiles, birds, and mammals) develop first a pronephros, and then a mesonephros, whereas the permanent kidney is a new organ, the metanephros. Whether these glands represent modifications of an originally continuous organ, or whether they are three distinct structures, is undecided, but however this may be the promeso-, and metanephroi of amniotes develop successively, one caudad of the other, in the order named.

I. The Urinary Organs

The Pronephros. - When functional, the pronephros consists of paired, segmentally arranged pronephric tubules; one end of each tubule opens into the coelom, the other into a longitudinal collecting duct which drains into the cloaca (Fig. 128 A). Near the nephrostome (the funnel-like opening into the coelom), knots of arteries project into the coelom, forming glomeruli. These filter wastes from the blood into the coelomic fluid which is then taken up by the tubules and carried by ciliary movement into the excretory ducts.

The human pronephros is vestigial. It consists of about seven pairs of rudimentary pronephric tubules, formed as dorsal sprouts from the nephrotomes in each segment, from the seventh to the fourteenth, and perhaps from more cranial segments as well (Fig. 35 B). Yet the earliest tubules begin to degenerate before the last appear. The nodules hollow out and open into the coelom (Fig. 128 B). Dorsally and laterally, the tubules of each side bend backward and unite to form a longitudinal pronephric collecting duct (Fig. 12S B, A). Caudal to the fourteenth segment no pronephric tubules are developed, but the free end of the collecting duct, by a process of terminal growth, extends caudad beneath the ectoderm (and lateral to the nephrogenic cord) until it reaches the lateral wall of the cloaca and perforates it (Fig. 87). Thus are formed the paired primary excretory {prone phric) ducts. The pronephric tubules begin to appear in embryos of 1.7 mm., with nine or ten primitive segments; at 2.5 mm. (23 segments), all the tubules have developed and the primary excretory duct is nearly complete. In 4.3 mm. embryos, the duct has reached the wall of the cloaca and soon after fuses with it (Fig. 87). The pronephric tubules promptly degenerate, but the primary excretory ducts persist and become the ducts of the mesonephroi.

Fig. 127. Transverse section of a 2.4 mm. human embryo, showing the intermediate cell mass or nephrotome (Kollmann).

Fig. 128. Diagrams illustrating the development of the proncphric duct and proncphric tubules (Felix-Prentiss). A, represents a later stage than B.

The Mesonephros

The mesonephros, like the pronephros, consists essentially of a series of tubules, each of which at one end is related to a knot of blood vessels and at the other end opens into the primary excretory duct (Fig. 87). But the mesonephric tubule differs in two important respects: ( i) the glomerulus indents one end of the tubule, and excreta from the blood pass directly into its lumen; (2) the nephrostomes are transitory and never open into the mesonephric chamber. The mesonephric tubules arise just caudal to the pronephros and from the same general source, that is, the nephrotomes. Only a few of the more cranial tubules, however, are formed from distinct intermediate cell masses, for, caudal to the tenth pair of segments, this mesoderm fuses into unsegmented, paired nephrogenic cords which may extend as far as the twenty-eighth segment (Fig. 132). The primary excretory ducts lie lateral to the nephrogenic cords.

When the developing mesonephric tubules begin to expand, there is not room for them in the dorsal body wall, which as a result bulges ventrally into the eoelom. Thus is produced on each side of the dorsal mesentery a longitudinal urogenital fold, which may extend from the sixth cervical to the third lumbar segment (Fig. 143). Later, this ridge is divided into a lateral mesonephric fold and into a median genital fold, the anlage of the genital gland (Figs. 13 1 and 144).

Differentiation of the Tubules

The nephrogenic cord in 2.5 mm. embryos first divides into spherical masses of cells, the anlages of the mesonephric tubules. As many as four of these are formed in a single segment. Appearing first in the thirteenth to fifteenth segments, the anlages of the tubules differentiate both cranially and caudally. In 5.3 mm. embryos the cephalic limit is reached in the sixth cervical segment, and thereafter degeneration begins at this end (Fig. 130). Hence, the more cranial tubules overlap those of the pronephros. In 7 mm. embryos, the caudal limit is reached in the third lumbar segment.

Differentiation of the tubule anlages progresses in a cephalo-caudad direction (Fig. 130). First, veSicles with lumina are formed (4.3 mm.) from the spherical masses (Fig. i2g AB). Next, the vesicles elongate laterally, unite with the primary excretory ducts, and become S-shaped (Fig. 129 B, C). The free, vesicular end of the tubule enlarges, becomes thin walled, and into this wall grows a knot of arteries to form the glomerulus (5 to 7 mm. ; Fig. 129 D). The wall of the vesicle about the glomerulus is Bowman 's capsule and the two constitute a renal corpuscle of the mesonephros (Fig. 131). The tubule, which was at first solid, is now lined with a low columnar epithelium. In the human embryo the tubules do not branch or coil as in the pig, consequently the mesonephros is relatively smaller. At 10 mm., about 35 tubules are present in each mesonephros and the glomeruli are conspicuous (Fig. 130). Each tubule shows a distal secretory portion and a proximal collecting part which connects with the duct (Fig. 13 1). The glomeruli form a single median column in the gland; the tubules are dorsal and the duct is lateral in position. Ventro-lateral branches from the aorta supply the glomeruli (Fig. 212), while the posterior cardinal veins (Fig. 145), dorsal in position, break up into a network of sinusoids about the tubules.

Fig. 129. Diagrams showing the differ-

Fig. 130. The anlages of the urinary or entiation of a mesonephric tubule (Felix- gans in a 10 mm. human embryo, as seen Prentiss). L. lateral; M. median. from the left side (adapted by Prentiss).

The pronephric duct, now termed the mesonephric, or Wolffian duct, is solid in 4.3 mm. embryos. A lumen is formed at 7 mm., wider opposite the openings of the tubules. The duct is important, as the ureteric anlage of the permanent kidney grows out from its caudal end, while the tube itself is transformed into the chief genital duct of the male.

That the human mesonephros is a functional excretory organ is plausible (Bremer, 1916), but not proved. Degeneration proceeds rapidly in embryos between 10 and 20 mm. long, beginning cranially (Fig. 148). New tubules are formed at the same time caudally (Fig. 130). In all 83 pairs of tubules arise, of which only 26 pairs persist at 21 mm., and these are usually broken at the angle between the collecting and secretory regions. How the genital system utilizes them for new purposes will be traced in a later section (p. 156).

The Metanephros

The essential parts of the permanent kidney are the renal corpuscles (glomeruli with Bow^man - s capsules), secretory tubules, and collecting tubules. Like the mesonephros, the metanephros is of double origin. The ureter, pelvis, calyces, and collecting tubules are outgrowths of the mesonephric duct. The secretory tubules and the capsules of the renal corpuscles are differentiated from the isolated, caudal end of the nephrogenic cord and thus have an origin similar to that of the mesonephric tubules.

In embryos of about 5 mm. the mesonephric duct makes a sharp bend just before it joins the cloaca, and it is at this angle that the ureteric evagination appears, dorsal and somewhat median in position (Fig. 139 B, C). The bud grows at first dorsally, then cephalad. Its distal end expands and forms the primitive pelvis; its proximal elongated portion is the ureter. The pelvic anlage grows into the lower end of the nephrogenic cord (Fig. 132), which, during the third month, becomes separated from the cranial end. The nephrogenic tissue forms a cap about the primitive pelvis, and, as the pelvis grows cranially, is carried along with it.

Fig. 132. Reconstruction of the metancphric anlages in a human embryo of about 9 mm (after Schreiner).

In embryos of g to 13 mm. the pelvis, having advanced cephalad through three segments, attains a position in the retroperitoneal tissue dorsal to the mesonephros and opposite the second lumbar segment. Thereafter, the kidney enlarges both cranially and caudally without shifting its mean position (Fig. 154).

Fig. 133. Diagrams showing the development of the primitive pelvis, calyces and collecting tubules of the metanephros (adapted by Prentiss).

Differentiation of the Ureteric Anlage

Primary collecting tubules grow out from the primitive pelvis in 10 mm. embryos. Of the first two, one is cranial, the other caudal in position, and, between these, two others usually appear (Fig. 133 B, C). From an ampullary enlargement, at the end of each primary tubule sprout off two, three, or four secondary tubules. These in turn give rise to tertiary tubules (Fig. 133 D) and repeated until the fifth month of fetal life, when it is estimated that twelve generations of tubules have been developed.

The pelvis and the primary and secondary tubules enlarge greatly during development. The two primary expansions become the major calyces, and the secondary tubules opening into them form the minor calyces (Fig. 134). The tubules of the third and fourth orders are taken up into the walls of the enlarged secondary tubules so that the tubules of the fifth order, 20 to 30 in number, open into the minor calyces as papillary ducts. The remaining orders of tubules constitute the collecting tubules which form the greater part of the medulla of the adult kidney.

When the four to six primary tubules develop, the nephrogenic cap about the primitive pelvis is subdivided and its four to six parts cover the end of each primary tubule.

As new orders of tubules arise, each mass of nephrogenic tissue increases in amount and is further subdivided until finally it forms a peripheral layer about the tips of the branches tributary to a primary tubule. The converging branches of such a tubular - tree - constitute a primary renal unit, or pyramid, with its base at the periphery of the kidney and its apex projecting pnto the pelvis. The apices of the pyramids are termed renal papilla:, and through them the papillary ducts open. The nephrogenic tissue forms the cortex of the kidney, and each subdivision of it, covering the tubules of a pyramid peripherally, is marked off on the surface of the organ by grooves or depressions. The human fetal kidney is thus distinctly lobed, the lobations persisting for several years after birth; this condition is permanent in reptiles, birds, and some mammals (whale; bear; ox). The primary pyramids are subdivided into several secondary and tertiary pyramids. Between the pyramids, the cortex of nephrogenic tissue dips down to the pelvis, forming the renal columns (of Bertin). The collecting tubules, on the other hand, extend out into the cortex as the cortical rays, or pars radiata of the cortex. In these rays, and in the medulla of the kidney, the collecting tubules run parallel and converge to the papillae.

Fig. 134. Reconstruction of the ureter, pelvis, calyces and their branches from a 16 mm. human embryo (Huber). X 50.

Differentiation of the Nephrogenic Tissue

In stages from 13 to 19 mm., the nephrogenic tissue about the ends of the collecting tubules condenses into spherical masses that lie in the angles between the buds of new collecting tubules and their parent stems (Fig. 135). One such metanephric sphere is formed for each new tubule. The spheres are converted into vesicles with eccentrically jilaced lumina. The vesicle elongates, its thicker outer wall forming an S-shaped tubule which unites with a collecting tubule, its thin inner wall becoming the capsule (Bowman's) of a renal corpuscle.

The uriniferous tubules of the adult kidney have a definite and peculiar structure and arrangement (Fig. 136 4 l). Beginning with a renal corpuscle, each tubule forms a proximal convoluted portion, a U-shaped loop (of Henle) with descending and ascending limbs, a connecting piece, which lies close to the renal corpuscle, and a distal convoluted portion continuous with the collecting tubule. These parts are derived from the S-shaped anlage, which is composed of a lower, middle, and upper limb. The middle limb, somewhat U-shaped, bulges into the concavity of Bowman's capsule (Fig. 136 B). By differentiation the lower portion of the lower limb is converted into Bowman's capsule, and ingrowing arteries form the glomerulus (Fig. 136 B, C). The upper part of the same limb by enlargement, elongation, and coiling becomes the proximal convoluted tubule. The neighboring portion of the middle limb forms the primitive loop (of Stoerck) ; the base of the middle limb gives rise to the connecting piece, and the rest of it, rvith the upper limb of the S, comprises the distal convoluted tubule. The primitive loop of Stoerck includes both the descending and ascending limbs of Henle's loop and a portion of the proximal convoluted tubule as well. Henle's loop is differentiated during the fourth fetal month and extends from the pars radiata of the cortex into the medulla (Fig. 137). The concavity of Bowman's capsule, into which grow the arterial loops of the glomerulus, is at first shallow. Eventually, the walls of the capsule grow about and enclose the vascular knot, except at the point where the arterioles enter and emerge (Fig. 135,4 and 5). Renal corpuscles are first fully formed at the end of the second month. The newer corpuscles differentiate peripherally from persisting nephrogenic tissue, and this may continue for some time after birth; hence, in the adult, the oldest corpuscles are those next the medulla. Reconstructions of the various stages in the development of the uriniferous tubules are shown in Fig. 138.

Fig. 135. (Huber). The left half of each figure represents an earlier stage than the right half.

Fig. 136. Diagrams showing the differentiation of the various parts of a human uriniferous tubule (adapted by Prentiss). A, From an adult; B, C, from embryos.

Fig. 137. Diagram showing the relation of Bowman's capsule and the uriniferous tubule to the collecting tubules of the metanephros (Huber), c, Collecting tubule; e, end branches of collecting tubules; r, renal corpuscles; n, neck; pc, pro.ximal convoluted tubule; dl,al, descending and ascending limbs of Henle - s loop, /; dc, distal convoluted tubule; j, junctional tubule.

Fig. 138. Reconstructed stages in the development of the human metanephric tubule at the seventh month (Huber). X 16.


The kidneys may fail to ascend from their embryonic position in the pelvis. Absence of one kidney is not infrequent. The kidneys sometimes fuse, either completely into a disc-shaped mass, or partially by cortical union (V/cm - -i/zoe ; in such cases the ducts usually are bilateral. Double or cleft ureters and pelves occur.

Renal Cysts (cystic kidney) result from the primary non-union of uriniferous and collecting tubules, or by the cystic degeneration of secondarily detached tubules (Kampmeier, 1923).

Differentiation of the Cloaca

In embryos of 1.4 mm., the cloaca, a caudal expansion of the primitive entodermal canal, is in contact ventrally with the ectoderm, and the area of union constitutes the cloacal membrane (Fig. 139 A). This membrane at first extends from the tail bud to the body stalk (Figs. 71 and 95), and occupies a region corresponding to the hind end of the primitive streak (Figs. 44 and 58). Later, Its expanse is diminished in both directions (Figs. 96, 141 and 142). Ventro-cephalad, the cloaca gives off the allantoic stalk, receives the mesonephric ducts laterally, and is prolonged caudally as the tail-gut (Fig. 139 B).

Fig. 139. Reconstructions of the early human cloaca (Pohlman-Prentiss). X about 50. .4 3.5 mm.; B, 4 mm.; C, 5 mm.; D, 7 mm.

The saddle-like partition between the intestine and allantois grows caudally, dividing the cloaca into a dorsal rectum and ventral, primitive urogenital sinus (Figs. 139 to 142). The division is complete in embryos of u to 15 mm., and at the same time the partition, fusing with the cloacal membrane, divides it into the anal membrane of the gut and the urogenital membrane (Fig. 142). The intermediate tissue represents the body of the primitive perineum. At u mm., according to Felix, the primitive urogenital sinus by elongation and constriction is differentiated into two regions: (1) a dorsal vesico-urethral anlage which receives the allantois and mesonephric duct, and is connected by the constriction with (2) the phallic portion (Figs. 140 and 141). The latter extends into the phallus of both sexes and forms a greater part of the male urethra (Fig. 142), as described on p. 164.

Fig. 140. Reconstruction from a 12 mm. human embryo, showing the partial division of the cloaca into rectum and urogenital sinus (Pohlman-Prentiss). X 65.

Fig. 141. Reconstruction of the caudal portion of an 11.5 mm. human embryo, showing the differentiating rectum, bladder and urethra (Keibel-Prentiss). X 25.

The vesico-urethral anlage enlarges and transforms into the bladder and into either the entire female urethra or the prostatic and membranous male urethra. In 7 mm. embryos the proximal ends of the mesonephric ducts are funnel shaped, and, at 10 mm., eoincident with the enlargement of the bladder, these ends are taken up into its wall until the ureters and mesonephric ducts acquire separate openings (Figs. 141 and 142). The ureters, having previously shifted their openings into the mesonephric ducts from a dorsal to lateral position, now open into the vesico-urethral anlage lateral to the mesonephric ducts. The lateral walls of the bladder anlage grow more rapidly than its dorso-median urethral wall; hence the ureters are carried cranially and laterally upon the wall of the bladder while the mesonephric ducts open close together on a hilloek, Muller's tubercle, into the dorsal wall of the urethra (Fig. 142). Thus an area, roughly bounded by the openings of the ureters and the mesonephric (ejaculatory) ducts, is mesodermal. Besides the trigone of the bladder the area includes a proximal segment of the urethra (Fig. 160 C). In the male, this stretch corresponds to the upper portion of the prostatic urethra; in the female, it includes much of the shorter definitive urethra.

Fig. 142. Reconstructions of the caudal end of a two-months - human embryo, showing the complete separation of the rectum and urogenital sinus and the relations of the urogenital ducts (Keibel-Prentiss). X 15.

The narrowed apex of the bladder, continuous with the allantoic stalk at the umbilicus, is known as the urachus (Fig. 157). It persists as the solid, fibrous middle umbilical ligament (Fig. 199). Contrary to earlier views, the allantois contributes nothing to the bladder or urachus (Felix, 1912).

The transitional epithelium of the bladder appears at 10 weeks. The circular and outer longitudinal layers of muscle develop at the end of the second month. The inner longitudinal muscle layer is found at 10 weeks and the sphincter vesicse in fetuses of three months.


A conspicuous malformation is that of a persistent cloaca, due to the failure of the rectum and urogenital sinus to separate. The bladder sometimes opens widely onto the ventral body wall and is everted through the fissure; a urogenital aperture corresponding to the upper extent of the primitive cloacal membrane would cause this condition (Fig. 139 C, D). At times, the urachus remains a patent tube, opening at the umbilicus as a urinary fistula. Portions of its epithelium which fail to degenerate may form cysts.

Fig. 143. Reconstruction of the male urethra and associated parts, from a fetus of four months (after Broman). X 13.

Accessory Genital Glands

The prostate gland develops in both sexes as outgrowths of the urethra, both above and below the entrance of the male ducts (Fig. 143). Hence, the upper portion, at least, must be mesodermal in origin. The tubules arise at ten weeks in five distinct groups and total an average number of 63. The surrounding mesenchyme differentiates both connective tissue and smooth muscle fibers, into which the anlages of the prostate grow. In the female, the homologue is rudimentary; these isolated para-urethral ducts (of Skene) number at most three.

The bulbo-urethral glands (of Cowper) arise in male embryos of nine weeks as solid, paired epithelial buds from the entoderm of the urethra (Fig. 143). The buds penetrate through the mesenchyme of the corpus cavernosum urethree, about which they enlarge. The glands branch, and, at four months, the epithelium becomes glandular. The vestibular glands (of Bartholin) are the homologues in the female of the bulbo-urethral glands. They appear at the same age as the male glands, grow until after puberty, and degenerate after the climacterium.

II. The Genital Organs

A. Indifferent Stage

The Gonads

In origin and early development, the ovary and testis are identical. The urogenital fold (p. 135) is the anlage of both the mesonephros and the genital gland (Figs. 392 and 144). At first two-layered, its epithelium in embryos of 5 mm. thickens over the ventro-median surface of the fold, becomes rciany-layered, and bulges into the coelom ventrally to produce the longitudinal genital fold (Fig. 13 1). The genital fold thus lies mesial and parallel to the mesonephric fold. Large primordial germ cells are found in the entoderm of the future intestinal tract; at 3.5 mm., these migrate into the dorsal mesenteric epithelium and thence into the epithelium of the genital fold. It is undecided whether or not the definitive germ cells of the genital glands are descendants of such elements. At 10 to 12 mm., the genital anlage shows no distinctive sexual differentiation (Fig. 145); there is a superficial eph/je/ia/ layer and an inner epithelial mass of somewhat open structure owing to the great development of the suprarenal glands and metanephroi, the cranial portions of the urogenital folds, at first parallel and close together, are displaeed laterally. This produces a double bend in each fold, which, in 20 mm. embryos, shows a cranial longitudinal portion, a transverse middle portion between the bends, and a longitudinal caudal portion (Fig. 160 A). In the last-named segment, the mesonephric ducts course to the urogenital sinus, and here the right and left folds fuse, producing the genital cord (h^ig. 154). As the genital glands increase in size, they become constricted from the mesonephric fold by lateral and mesial grooves until the originally broad base of the genital fold is converted into a stalk (Figs. 149 to 151). This mesenterial attachment extends lengthwise and forms in the male the mesorchium , in the female the mesovarium.

Fig. 144. Ventral view of the urogenital folds in a human embryo of 9 mm. (Kollmann).

Fig. 145. Transverse section through the mesonephros, genital gland and suprarenal gland of the right side; from a 12 mm. human embryo (Prentissf. X 165.

The Primitive Genital Ducts

The mesonephric ducts, with the degeneration of the mesonephroi, become the male genital ducts; their origin and early history have been deseribed (pp. 137 and 139).

Both sexes also develop a pair of female ducts. In embryos of 10 mm., these Mullerian ducts arise as thickened ventro-lateral grooves in the urogenital epithelium, near the eranial ends of the mesonephroi (Fig. 146 A).

Fig. 146. Transverse sections through the anlage of the right Mullerian duct from a 10 mm. human embryo (Prentiss). X 250. A, Cranial end of groove; B, three sections caudad.

Fig. 147. Ventral dissection of an 18 mm. pig embryo, to show the growing Mullerian ducts (Prentiss). X 7.

Fig. 148. Ventral dissection of a 24 mm. pig embryo, showing a later stage in growth of the Mullerian ducts (Prentiss). X 6.

Fig. 149. Section through the left testis and mesonephros of a 20 mm. human embryo (Prentiss). X 250.

Caudally, the dorsal and ventral lips of the groove close and form a tube which separates from the epithelium and lies beneath it (Fig. 146 B). Cranially, the tube remains open as the funnel-shaped ostium abdominale of the Mullerian duct. The solid end of the tube grows caudalward, beneath the epithelium and lateral to the mesonephric, or male ducts (Figs. 147 to 149). Eventually, by way of the genital cord, the Mullerian ducts reach the median dorsal wall of the urogenital sinus and open into it (Figs. 142 and 160 A). In the lov/est vertebrates, the Mullerian duct arises by a longitudinal splitting of the mesonephric duct.

Embryos not longer than 12 mm. are thus characterized by the possession of indifferent genital glands and both male and female genital ducts. There is as yet no sexual differentiation.

B. Internal Sexual Transformations

Differentiation of the Testis. - In male embryos of 13 mm., the genital glands show two characters which mark them as testes: (1) the occurrence of branched, anastomosing cords of cells, the testis cords; (2) the occurrence between epithelium and testis cords of a layer of tissue, the anlage of the tunica albuginea (Fig. 149). According to Felix (1912), the testis cords of man are developed suddenly from the loose, inner epithelial mass by a condensation of its cells; on the contrary, xMlen (1904) holds that in the pig and rabbit they grow in from the surface epithelium The cords converge towards the mesorchium, where they form the dense, epithelial anlage of the slenderer rete testis. Two or three layers of loosely arranged cells between the testis cords and the epithelium constitute the future tunica albuginea.

Fig. 150. Section through the left testis of a fetus of fourteen weeks (Prentiss). X 44.

The testis cords soon become rounded and are marked off by connective-tissue sheaths from the intermediate cords, which are columns of undifferentiated tissue lying between them (Fig. 150). Toward the rete testis, the sheaths of the testis cords unite to form the anlage of the mediastinum testis. The testis cords are composed chiefly of indifferent cells, with a few larger germ cells. The cells gradually arrange themselves radially about the inside of the conneetive-tissue sheath as a many-layered epithelium; during the seventh month, a lumen ajDpears and extends toward the rete testis to meet lumina which have formed there. Thus the solid cords of both are converted into tubules. The distal portions of the testis tubules anastomose and form the tuhnli contorti. Their proximal portions remain straight, as the tuhuli recti. The rete testis beeomes a network of small tubules that finally unite with the efferent duetules.

The primordial germ cells of the testis eords form the spermatogonia of the seminiferous tubules, and from these, at puberty, are probably developed the later generations of spermatogonia, although some claim that the early germ cells all disappear, to be replaced later from the indifferent elements. The indifterent cells of the tubules become the sustcntacnlar cells (of Sertoli) of the adult testis. Certain cells of the intermediate cords, epithelial in origin, are transformed into large, pale cells, which, after puberty, are numerous in the interstitial connective tissue and hence are designated interstitial cells. The intermediate cords, as such, disappear, I:)ut the connective-tissue sheaths of the tubules unite to form septula which extend from the mediastinum testis to the fibrous tunica albuginea.

Differentiation of the Ovary

The primitive ovary, like the testis, consists of an inner epithelial mass, bounded by the parent peritoneal epithelium. The ovarian characters ajjpear much more slowly than those of the testis. In fetuses of ten to eleven weeks, the inner ei^ithelial mass, composed of indifferent cells and primordial germ cells, becomes less dense centrally and bulges into the mesovarium (Fig. 151). There may be distinguished a dense, outer cortex beneath the epithelium, a clearer medullary zone containing large germ cells, and a dense, cellular anlage in the mesovarium, the primitive rete ovaru, which is the homologue of the rete testis. Neither epithelial cords nor tunica albuginea are developed at this stage, as in the testis.

Later, three important changes take place: (1) There is an ingrowth of connective tissue and blood vessels from the hilus, resulting in the formation of mediastinum and septula. (2) Most of the cells derived from the inner epithelial mass are transformed into young ova, the process extending from the rete ovaru peripherally (Fig. 151). (3) In fetuses of three to five months, the ovary grows rapidly, owing to the formation of a new peripheral zone of cells, derived perhaps in part from the peritoneal epithelium. At the end of this period the septula line the epithelium with a fibrous sheath, the anlage of the tunica albuginea. Hereafter, such folds of the epithelium as form do not penetrate beyond the tunica albuginea and all cells derived from this source subsequently degenerate. This new peripheral zone, according to Felix, is always a single cellular mass in man, cords, or - Pfluger - s tubes, - never growing in from the epithelium. Generally, it has been believed that the primary follicles are derived from the subdivision of such cords.

Fig. 152. Section through the ovarian cortex of a five-months' fetus (De Lee).

Fig. 151. Section through the left ovary of a three-months - fetus (Prentiss). X 44.

Coincident with the origin of a new zone of cells at the periphery of the ovary, goes the degeneration of young ova in the medulla. Invading connective tissue separates these germ cells into clusters, or cords, which degenerate and leave only a stroma of fibrous tissue in the medulla. Late in fetal life, indifferent cells, by surrounding the young ova of the cortex, produce primordial follicles (Fig. 13 A) whose differentiation into vesicular follicles is described in an earlier chapter (p. 20). In opposition to this classic concept, Allen ( 1923) and others contend that the definitive ova do not represent grown primordial ones but that they are new cells proliferated periodically in the adult from the germinal (peritoneal) epithelium.


Congenital absence or duplication of the testes and ovaries is very rare. Fused testes and lobed ovaries are also known.

Teratomata . - These peculiar tumor-like growths occur rather frequently in the ovarv, less often in the testis and other regions. The simpler types, called dermoid cysts, contain such ectodermal derivatives as skin, hair, nails, teeth, and sebaceous glands. They grade into comple.Kes consisting of organ-like masses, from all three germ layers, intermingled without order. Misshapen representatives of all tissues and organs may be present. Among other explanations of the cause, the isolation and subsequent faulty development of blastomeres has been advanced.

Transformation of the Mesonephric Tubules and Ducts

In both male and female embryos of 21 mm., the mesonephros has degenerated until only twenty-six tubules at most persist, and these are separated into a cranial and a caudal group. In the cranial group of 5 to 12 tubules, the collecting jiortions have broken apart from the secretory portions. The free ends of these collecting tubules project against that part of the inner epithelial mass which gives rise to the rete tubules of either testis or ovary (Figs. 149 and 151). The cords of the rete develop in contact with the collecting tubules of the mesonephros and unite with them in fetuses of 10 weeks.

In the male, the lumina of rete and collecting tubules become continuous and the cranial collecting group is transformed into the ductuli efcrentcs of the epididymis. During the fifth month of pregnancy the efferent ductules coil at their proximal ends, and, when surrounded by connective tissue, they are known as lobuli epididymidis. The lower group of collecting tubules persist as the vesitigial paradidymis and duciuli abherautes (Fig. 160 G). The efferent ductules convey spermatozoa from the testis tubules into the mesonephric duct, which thus becomes the male genital duct. The cranial portion of the mesonephric duct coils and forms the ductus epididymidis; its blind cranial end persists as the appendix epididymidis. The caudal portion of the male duct remains straight, and, as the ductus deferens and ejaculatory duct, extends from the epididymis to the urethra. Near its opening into the latter it dilates to form the ampulla, from the wall of which is evaginated the sacculated seminal vesicle in fetuses of three months (Fig. 143).

In the female, the rete ovaru is always vestigial, yet some time before birth it becomes tubular and unites with the cranial persisting group of mesonephric collecting tubules which forms a rudimentary structure, the epodphoron (Fig. 160 B). The caudal group of mesonephric tubules constitutes the paroophoron. Usually the greater part of the mesonephric ducts atrophy in the female, the process beginning early in the third month, but portions persist as Gartner - s ducts of the epodphoron.

Gartner's ducts may extend as vestigial structures from the epodphoron to the lateral walls of the vagina, passing through the broad ligament and the wall of the uterus. They open into the vagina close to the free border of the hymen. The ducts are rarely present throughout their entire length and are absent in two-thirds to three-quarters of the cases examined.

Transformation of the Mullerian Ducts

The Mullerian, or female ducts, follow the course of the mesonephric ducts (Fig. 148). At first lateral in position, the Mullerian ducts cross the mesonephric ducts and enter the genital cord median to them (Fig. 160 A). In embryos of two months their caudal ends are dorsal to the urogenital sinus and extend as far as the Mullerian tubercle, a projection into the median dorsal wall of the primitive urethra formed by the earlier entrance of the mesonephric ducts (Fig. 142). This tubercle marks also the position of the future hymen. In fetuses of u weeks the Mullerian ducts break through the wall of the urethra and open into its cavity. Before this takes place, their caudal ends, which are pressed close together between the mesonephric ducts in the genital cord, fuse, and in both male and female embryos of two months give rise to the single anlage of the uterus and vagina (Figs. 142 and 153 A). The paired cranial portions of the Mullerian ducts become the uterine tubes. During development, the ostial ends of the uterine tubes undergo a true descensus from the third thoracic to the fourth lumbar vertebra.

In the male, these parts are rudimentary. Those portions of the Mullerian ducts corresponding to the uterine tubes and uterus begin to degenerate at the beginning of the third month. The vaginal segment remains as a pouch on the dorsal wall of the urethra, the vagina masculina, or prostatic utricle (Fig. 143). The extreme cranial end of each Mullerian duct constitutes a so-called appendix testis (Fig. 160 C).

The Uterus and Vagina

Since the Mullerian ducts develop in the urogenital folds, they make two bends in their course (Fig. 153 A) corresponding to those of the folds (p. 150). Each duct consists of a cranial longitudinal portion, a middle transverse portion, and a caudal longitudinal portion which is fused with its fellow to form the utero-vaginal anlage. At the angle between the cranial and middle segments is attached the inguinal fold, the future round ligament of the uterus (Figs. 154 and 155). The mesenchyme condenses about the utero-vaginal anlage and the middle transverse portion of the Mullerian ducts, forming a thick, sharply defined layer, from which is differentiated later the muscle and connective tissue of these organs (Fig. 153 A). As development proceeds, the cranial wall between the transverse limbs of the Mullerian ducts bulges outward, so that its original cranial concavity becomes convex (Fig. 153 B). The middle, transverse portions of the ducts are thus taken up into the wall of the uterus to form its JunJus, while the narrow cervix of the uterus and the vagina arise from the original utero-vaginal anlage. A distinction between uterus and vagina is not evident until the middle of the fourth month. The entrance to the vagina is originally some distance above the outlet of the urogenital sinus (Fig. 160 A). This intervening stretch of sinus hereafter elongates relatively little and so becomes the shallow vestibule into which both urethra and vagina open (Fig. 160 B),

Fig. 153. Diagrams illustrating the development of the uterus and vagina (Felix-Prentiss).

The lower limit of the vagina lies at the level of Muller' s tubercle, where the utero-vaginal anlage breaks through the wall of the urogenital sinus. The tubercle is compressed into a disc, lined internally by the vaginal epithelium and externally by the epithelium of the urogenital sinus, or future vestibule. These layers, with the mesenchyme between them, constitute the hymen, which thus guards the opening into the vagina (Fig. 160 A, B). A cireular aperture in the hymen is for a time closed by a knob of epithelial cells, but later, when the hymen becomes funnel-shaped, the opening is compressed laterally to form a sagittal slit.

Muller's tubercle persists in the male as the colliculus seminalis, from the summit of which leads off the prostatic utricle.

At 10 weeks, the serosa, muscularis, and mucosa are indicated. The first circular muscle fibers appear during the fifth month; the other muscle layers develop later. The epithelium of the uterine tubes and corpus remains simple; that of the cervix and vagina becomes stratified at nine weeks. The tubular glands of the corpus appear about the seventh month. The uterus shortens greatly soon after birth and does not fully recoup this loss until the eleventh year. The virginal size is attained by a short period of rapid grovdh, chiefly before puberty. The vagina is for a time without a lumen, and solid epithelium fills its fornices. The vaginal lum.en reappears in fetuses of about five months through degeneration of the central epithelial cells.


Many cases of abnormal uterus and vagina occur. The more common anomalies are: (1) Complete duplication of the uterus and vagina, due to the failure of the Mullerian ducts to fuse. (2) Uterus bicornis, due to the incomplete fusion of the ducts. Combined with these defects, the lumen of the uterus and vagina may fail, partly or completely, to develop and the vaginal canal may not open to the exterior (imperforate hymen) . (3) The body of the uterus may remain flat (uterus planifundus; Fig. 153 . 1 ) or fail to grow to normal size (uterus fetalis and infantalis). (4) Congenital absence of one or both uterine tubes, or of the uterus or vagina, rarely occurs, but may be associated with hermaphroditism of the external genitalia. The hymen is of variable shape.

Fig. 154. Ventral dissection of the urogenital organs in a human embryo of two months (Prentiss). The right suprarenal gland has been removed to show the metanephros.

Ligaments of the Internal Genitalia

Female. - The ovary is primarily suspended by a short mesentery, termed the mesovarium (Fig. 151). A further support is furnished by the terminal portion of the primitive genital fold, which unites the caudal end of the ovary first to the genital cord and then to the uterus that develops in it. This connection becomes fibrous and is known as the proper ligament of the ovary (Fig. 15 s). With the degeneration of the mesonephric system, the uterine tube lies in a fold, the mesosalpinx (Fig. 151).

The mutual fusion of the caudal portions of the urogenital folds, as the genital cord, forms a mesenchymal shelf bridging in the coronal plane between the two lateral body walls and containing the uterus in its center (Fig. 154). It persists as the sheet-like broad ligaments of the uterus.

In embryos of 14 mm., a band, called the inguinal fold, joins the urogenital fold to the inguinal crest, which is merely a prominence on the adjoining al)dominal wall (Fig. 154). Within the inguinal crest is differentiated the chorda gubcrnaculi, which later becomes fibrous. The abdominal muscles develop around it and form a tube, the inguinal canal. At the outer end of the canal the external obliciue muscle leaves a foramen, through which the chorda connects with a second cord that extends to the genital swelling and is hence designated the ligamentum labiale. The chorda gubernaculi and the ligamentum labiale thus form a continuous cable from the labium majus to the uterus, which in the meantime has been developing in the fused urogenital folds; the two together constitute the round ligament of the uterus (F'ig. 155).

Fig. 155. Ventral dissection of the urogenital organs in a female fetus of nine weeks (Prentiss).

Male. - The primitive mesentery of the testis is the mesorchium (Figs. 149 and 150). It is represented in the adult as the fold between the epididymis and testis. The degenerating cephalic end of the mesonephros for a time constitutes the so-called diaphragmatic ligament of the mesonephros (Figs. 154 and 155).

The ligamentum testis, like the ligamentum ovaru, develops in the lower end of the genital fold and extends from the caudal pole of the testis to the mesonephric fold at a point adjacent to the attachment of the bridgelike inguinal fold (cf. Fig. 155). As in the female, the inguinal fold connects with the chorda guhernaculi within the inguinal crest, and this in turn is continued by way of the liganieutum scroti to the integument of the scrotum. A cord differentiates in the mesonephric fold and unites the ligamentum testis to the chorda gubernaculum. Thus there is formed a continuous ligament, the gubernaculum testis, extending from the caudal end of the testis through the inguinal canal to the scrotal integument. The gubernaculum is composed of the ligamentum testis, a mesonephric cord, the chorda gubernaculi, and the ligamentum scroti. It is the homologue of the ovarian ligament plus the round ligament of the uterus, between which the uterus intervenes (Fig. 155).

Descent of the Testis and Ovary

The original positions of the testis and ovary change during development. At first they are elongate structures, extending in the abdominal cavity from the diaphragm toward the pelvis (Fig. 144). Since their caudal ends continue to grow and enlarge while their cranial portions atrophy, there is a progressive, wave-like shifting of the glands caudad. Yet an actual internal descent by mass movement does not occur. When the process of growth and degeneration is complete, the caudal ends of the testes lie at the boundary line between the abdomen and pelvis, whereas the ovaries are located in the pelvis itself, a position which they retain. Owing to the rotation of the ovary about its middle point as an axis, it takes up a transverse position. The ovary also rotates nearly 180° about the Mullerian duct as an axis, and thus comes to lie caudal to the uterine tube.

Fig. 156. Diagrams illustrating the descent of the testis, p.v., Processus vaginalis: .v., obliterated vaginal sac.

In addition to its early apparent migration, the testes normally leave the abdominal cavity and descend bodily into the scrotum. At the beginning of the third month, while the testes are still fairly high in the abdomen, sac-like pockets appear in each side of the ventral abdominal wall. These are the anlages of the vaginal sacs, and during the fourth, fifth, and sixth fetal months the testes lie near them without change of position. Each processus {saccns) vaginalis evaginates over the pubis, through the inguinal canal, and into the scrotum (Fig. 156). During the seventh to ninth months the testes also descend rapidly along the same path (Fig. 157). Although the factors involved are not sufficiently understood, it is clear that the gubernaculum testis plays an important part. From the caudal pole of each testis the corresponding gubernaculum extends through the inguinal canal to the scrotal wall. During the seventh month the gubernaculum not only ceases growth but actually shortens one-half. The resultant relative and actual shortening serves to draw the testes into the scrotum (Fig. 157), where they usually are found by the ninth month, or at least before birth. It must be understood that the testis and gubernaculum are covered by the peritoneum before the descent begins; consequently the testis follows the gubernaculum along the inguinal canal dorsal to the peritoneum, and, when it reaches the scrotum, is invaginated into the processus vaginalis, but does not lie within the cavity of the coelomic extension (Fig. 156). The gubernaculum of a newborn is but one-fourth the length when descensus begins; after birth it atrophies almost completely within a few months after birth the narrow canal connecting the processus vaginalis with the abdominal cavity becomes solid and its epithelium is resorbed. The vaginal sac, now isolated, represents the tunica vaginalis of the testis. Its visceral layer is closely applied to the testis and its parietal layer forms the lining of the scrotal sac. The ductus deferens, and the spermatic vessels and nerves, are carried down into the scrotum with the testis and epididymis. They are surrounded by connective tissue and constitute the spermatic cord. Owing to the path taken by the testis, the ductus deferens loops over the ureter in the abdomen (Fig. 160 C).

Fig. 157. Ventral dissection of a full-term fetus to demonstrate the relation of the descended testis to the processus vaginalis (partly after Kollmann and Corning). On the left the peritoneum is intact; on the right the peritoneum and its sacculation are opened and the testis is rotated 90°.

In the female, shallow peritoneal pockets, frequently persistent as the diverticula of Nuck, correspond to the vaginal sacs of the male. Rarely, a more or less complete descent of the ovary into the labium majus occurs. The interposition of the uterus between the ovarian and round ligaments is responsible for the normal retention of the ovaries in the abdomen (Fig. 155).


At times, the testes remain in the abdomen, undescended, a condition known as cryptorchism and associated with sterility in man. In some mammals (whale; elephant) it is the normal condition. When the inguinal canals of man remain open, conditions are favorable for one type of inguinal hernia of the intestine. Open inguinal canals, with a periodic descent during the breeding season, occur normally in some animals (rodents; bats).

C. The External Genitalia

Recent investigation (Spaulding, 1921) proves that the external genitalia exhibit recognizable sex differences almost from their first appearance. In embryos of 8 mm., a rounded genital tubercle develops in the midline of the ventral body wall, between the umbilical cord and tail (Fig. 144). Its caudal slope bears the shallow urethral groove which is separated from the anal pit by a transverse ridge (Figs. 158 A and 159 A ) ; this ridge comprises the primitive perineum. The margins of the groove are slightly elevated as the urethral folds. Embryos of about 15 mm. show rupture of the urethral membrane in the floor of the groove, and the genital tubercle becomes more conical. Sex can now be recognized by the length of the urethral groove which in males extends from the base of the tubercle nearly to its apex (Fig. 158 A, B), whereas in females it is shorter and terminates some distance below the apex (Fig. 159 A, B)\ this diagnostic feature prevails until the definitive modelling begins.

At about 16 mm. (seven weeks) the genital tubercle has elongated into a somewhat cylindrical phallus, bearing at its tip the rounded glans which is set off by a constricted neck from the shaft-like body (Figs. 158 A and 159 A). On either side of the base of the phallus, and separated from it by a groove, are lateral, rounded ridges; these are the labioscrotal swellings, possibly represented much earlier by certain indefinite elevations.


Embryos of ten weeks are at the beginning of the definitive stage. In the male, the edges of the urethral groove progressively fold together and thus transform the open urogenital sinus into the tubular urethra (Figs. 158 B, C and 142). The fused edges constitute the raphe (Fig. 158 D). The scrotal swellings shift caudad to their final position where each becomes a half of the scrotum, separated from its mate by the raphe and underlying septum scroti. In the meantime, the shaft of the penis elongates, and, by the fourteenth week, the urethra has closed as far as the glans. The urethra is then continued along an epithelial plate which represents a solid part of the original urethral anlage incompletely partitioning the glans; by splitting, the plate is first converted into a trough which promptly recloses into a tube that continues the urethra to the definitive opening at the tip of the glans. A cylindrical collar of the surface epithelium, incomplete on the anal side, grows deep into the end of the primitive glans. By the disappearance of the central cells of the epithelial downgrowth, an outer cylindrical mantle, the prepncumi, or foreskin, is formed about the spheroidal glans penis (cf. Fig. 86). Where the epithelial downgrowth is incomplete the glans and fore-skin remain connected by the frenulum prepucu. The coropora cavernosa penis arise as paired mesenchymal columns. The corpus cavernosuni urethra: results from the linking of similar, unpaired anlages, one in the glans the other in the shaft.

Fig. 158. Stages in the development of the male external genitalia (redrawn after S]3aulding). . 1 , Nearly seven weeks (X 15); B, nearly eight weeks (X 12): C, ten weeks (X 8); D, twelve weeks ( X 8).

Fig. 159. Stages in the development of the female external genitalia (redrawn after Spaulding). , 4 , Nearly seven weeks (X i8); B, nearly eight weeks (X 15); C, ten weeks (X u): twelve weeks (X 8).


Changes in the female are less profound, yet slower (Fig. 159). The phallus lags in development and becomes the clitoris, with its homologous glans clitoridis and prepucium. The shorter urethral groove never extends onto the glans, as in the male. It remains open as the vestibule. The urethral folds which flank the original groove constitute the labia nuiwra. The primitive labio-scrbtal swellings grow caudad and fuse in front of the anus as the posterior commissure (embryos of u weeks), while the original lateral portions enlarge into the labia majora; these parts now form a horse-shoe shaped rim, open toward the umbilicus. The mans pubis, which arises later, appears to develop independently.

Besides the sexual difference in the length of the urethral groove already mentioned, male embryos of more than 20 mm. are characterized by a phallus which stands at right angles to the body, whereas in the female it curves downward.

Homologies of Internal and External Genitalia

Male .

Indifferent stage .

Female .

TestiS Rete testis.


Ovary (Rete ovaru).

Ligamentum testis. Gubernaculum testis.

Primitive ligaments.

Ligamentum ovaru proprium. Lig. ovaru -|- Lig. teres.

Ductuli efferentes. Paradidymis.

Mesonephric collecting tubules. Cranial group.

Caudal group.



Appendix epididymidis Ductus epididymidis. Ductus deferens. Seminal vesicle. Ejaculatory duct.

Mesonephric duct.

Gartner 's duct.

(1) Appendix testis.

(2) (3) Utriculus prostaticus (Vagina masculina).

Mullerian duct.

(0 Uterine tube.

( 2 ) Uterus.

(3) Vagina.

Colliculus seminalis.

Muller's tubercle. Hymen.

(1) Prostatic and membranous urethra.

(2) Cavernous urethra.

(3) Prostate gland.

(4) Bulbo-urethral glands.

Urogenital sinus.

(1) Urethra and vestibule.

(2) (3) Para-urethral ducts.

(4) Vestibular glands.


Gians penis.

Anal surface of penis.



Lips of urethral groove.


Gians clitoridis. Labia minora.


Labio-scrotal swellings.

Labia majora. Posterior commissure.

Epididymis .

Fig. 160. Diagrams to show the development of male and female genital organs from a common type (after Thompson).


If the lips of the slit-like urogenital opening on the under surface of the penis fail to fuse, hypospadias results. Rarely, there is a similar defect on the upper surface - epispadias; it is usually associated with vesico-abdominal fissure.

True hermaphroditism consists in the presence of both testis and ovary in the same invividual. It is of rare occurrence in birds and mammals, is not uncommon in the lower dertelirates, and is the normal condition in many invertebrates (worms; molluscs). In man there are five authentic cases with combined ovotestis and four cases with separate ovary and testis. The internal genitalia are faultily bisexual. The external genitalia show mixed male and female characteristics. The secondary sexual characters (beard, mamma, voice, etc.) are usually intermediate, tending now one way, now the other.

False hermaphroditism is characterized by the presence of the genital glands of one sex in an individual whose secondary sexual characters and external or internal genitalia resemble those of the opposite sex. In masculine hermaphroditism, an individual possesses testes, often undescended, but the external genitals (by retarded development) and secondary characters are like thoSe of the female. In feminine hermaphroditism, ovaries are present, and sometimes descended, but the other sexual characters, such as enlarged clitoris or fused laldae, simulate the male. The cause of hermaphroditism is unknown.

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   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Cite this page: Hill, M.A. (2024, February 28) Embryology Book - Developmental Anatomy 1924-8. Retrieved from

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